1. Field of the Invention
The present invention relates to an improved image-receiving sheet for thermal transfer printing in which a thermally sublimable dye is used. More particularly, the present invention relates to an improved image-receiving sheet for thermal transfer printing which has an improved intermediate layer containing fine particles of thermosetting resin and fine particles of polyolefin resin and which exhibits an improved recording sensitivity and provides excellent printed images with no missing dots.
2. Description of the Prior Art
Thermal printing systems in which printed images are obtained upon reception of input signals are made up of a relatively simple apparatus and are inexpensive and low in noise. In view of this, they have increasing utility in various fields such as facsimiles, terminal printers for electronic computers, printers for measuring instruments, video printers, and the like.
As the recording medium to be used in these thermal printing systems, there has been generally used a so-called spontaneous developing heat sensitive paper having a recording layer capable of causing a physical or chemical change upon application of heat to provide color development. However, the spontaneous developing heat sensitive paper of the color developing type has disadvantages in that it is liable to undesirably cause color development during the fabrication process or during storage; and the images printed on the paper are poor in storage stability and they are apt to fade on contact with organic solvents or chemicals.
In order to improve the above situation, there has been proposed a printing system wherein a recording medium in which a coloring material such as a dye or colorant is utilized is used instead of the foregoing spontaneous developing heat sensitive paper, for example, as disclosed in Japanese Unexamined Patent Publication Sho. 51(1976)-15446. In the printing system disclosed in this publication, a sheet comprising a substrate such as a paper or polymer film and a colorant layer containing the coloring material (which is in the solid or semi-solid state at ordinally temperature) being formed on the substrate is firstly provided, and this sheet and a recording sheet are superposed so as to make the colorant layer contacted with the recording sheet and heated from the non-faced side of the former sheet by a heating means such as a thermal head to transfer the coloring material in the colorant layer on the recording sheet upon the electric signals provided by the heating means, whereby images corresponding to image information are recorded on the recording sheet.
In the above printing system, the coloring material in the colorant layer is caused to melt, evaporate or sublimate by application of heat and transferred on the recording sheet, thereby forming a record image by adhesion, adsorption or reception of the coloring material on the recording sheet. In view of this, this printing system has been evaluated as being advantageous from the viewpoint that there can be used an ordinary paper (wood free paper) as the recording sheet. Further, as for this printing system, when a sublimable dye is used as the coloring material, there can be obtained a printed image excelling in tone reproduction. In order to develop this advantage in the full-color printing, various studies have been made.
However, there are disadvantages for the foregoing printing system wherein an ordinary paper (wood free paper) is used as the recording sheet in that sufficient dye-reception hardly occurs causing such printed images which are poor in color density (optical density) and whose image is markedly discolored as time lapses.
To avoid this, there has been proposed the use of an image-receiving sheet having an image-receiving layer containing a thermosetting resin as the main constituent which is formed on a substrate as disclosed in Japanese Unexamined Patent Publication Sho. 57(1982)-107885 or U.S. Pat. No. 3,601,484.
The use of this image-receiving sheet is effective in somewhat improving the recording sensitivity and storage-ability. However, there still remains a problem that when an ordinary paper (wood free paper) is used as the substrate, it is difficult to provide a desirable image-receiving layer containing a thermosetting resin as the main constituent of uniform thickness with the paper and thus, the resulting image-receiving sheet unavoidably becomes such that it is poor in recording sensitivity and provides undesirable images inferior in quality.
In order to eliminate the above problem, there has been proposed another image-receiving sheet having an intermediate layer comprising a thermosetting resin which is disposed between a substrate and an image-receiving layer as disclosed in Japanese Unexamined Patent Publication Sho. 60(1985)-236794 or Sho. 61(1986)-144394.
This image-receiving sheet is so designed that its image-receiving layer can be effectively contacted with the dye layer of the dye transfer sheet to prevent occurrence of negative phenomena such as air-gap upon printing, and it exhibits an improved recording sensitivity and provides improved record images.
In addition, there has been proposed a further image-receiving sheet having an intermediate layer containing fine particles of a specific polyolefin resin which is disposed between a substrate and an image-receiving layer by, among others, four of the coinventors of the present invention as disclosed in U.S. Pat. No. 4,837,200. The use of this image-receiving sheet is effective especially in obtaining desirable recorded images free of any missing transfer portion.